To produce plastic items, in particular polyurethane articles, the prior art makes use of a mould release agent which enables removal of articles from the mould. These are generally waxes, silicones or soaps, dissolved/dispersed in water and/or organic solvents. Extraordinary efforts have been made for a long time to reduce or completely eliminate the use of mould release agents. There is a number of reasons for this which are well known to a person skilled in the art:
cost of the mould release agent PA1 the expenditure of time and the extension of the working cycle as a result of applying the mould release agent PA1 the precautions which have to be taken when using a mould release agent (aerosols) PA1 costs of the subsequent removal of the mould release agent by processes such as "powerwash" or sandblasting etc. PA1 a) Fe:&lt;100 wt. % PA1 b) at least one element from the group PA1 c) at least one element from the group PA1 d) C: 0.1 to 1.5 wt. %, preferably 0.15 to 1.0 wt. % and optionally PA1 e) Co: 0 to 10 wt. %, preferably 2 to 8 wt. %, in particular 4 to 6 wt. % and other elements such as S, Ti, Ni, and optionally PA1 f) N: 0 to 10 wt. % of N in a nitriding layer. PA1 a) Fe:&lt;100 wt. % PA1 b) at least one element from the group W, Mo, V with a concentration of PA1 c) at least one element from the group Cr, Al with a concentration of PA1 d) optionally with carbon with a concentration of PA1 e) optionally with cobalt with a concentration of PA1 f) optionally with nitrogen with a concentration of PA1 A) polyisocyanates and PA1 B) compounds with at least 2 hydrogen items which are able to react with isocyanates and have a molecular weight of 60 to 2000, optionally in the presence of chain-lengthening and/or cross-linking agents, blowing agents and flier auxiliary agents and additives, characterised in that they are produced in moulds in which the internal metal surface(s) consist of steel with the percentage composition PA1 It is particularly surprising that even so called EMR polyurethanes, that is PUR resins which are processed using external mould release agents and contain no internal mould release agents, can now be processed without applying mould release agents. PA1 Equally surprising is the fact that expanded polyurethanes with low density can now be processed without applying mould release agents. PA1 Especially surprising is also the finding that the mould temperatures can be lowered considerably. As a result of this lowering of the mould temperature, the batch shrinkage process is intensified which facilitates removal of the article. PA1 It is surprising to a person skilled in the art that, according to the invention, steel known per se has this effect, whereas tests, including those of the applicant, on obtaining improved separation with chrome-plated or nickel-plated moulds failed (independently of the repair problems associated with this type of metallised surface). PA1 The solution according to the invention leads to a cost-effective production of plastic parts since application processes and post-treatments can be omitted entirely or to a large extent. PA1 1. The starting components are aliphatic, cycloaliphatic, arylaliphatic, aromatic and heterocyclic polyisocyanates, such as are described e.g. by W. Siefken in Justus Liebigs Annalen der Chemie, 562, pages 75 to 136, for example those of the formula EQU Q (NCO).sub.n PA1 in which PA1 a) Fe:&lt;100 wt. % PA1 b) at least one element from the group PA1 c) at least one element from the group PA1 d) C: 0.1 to 1.5 wt. %, preferably 0.15 to 1.0 wt. % also optionally PA1 e) Co: 0 to 10 wt. %, preferably 2 to 8 wt. %, in particular 4 to 6 wt. % and other elements such as S, Ti, Ni, and optionally PA1 f) N: 0 to 10 wt. % of N in a nitriding layer. PA1 Lehrheft zum Selbststudium der DAG-Technikum, Essen 1982, Werkstoffkunde, Volume 4, page 34 et seq.; PA1 Lehrheft zum Selbststudium der DAG-Technikum, Essen 1982, Werkstoffkunde, Volume 2, page 25 et seq.; PA1 Fachkunde Metall, Europa-Verlag, 48th edition 1987; PA1 Tabellenbuch Metall, Europa-Verlag, 38th edition, p.106; PA1 Werkstoffkunde fur Praktiker, Europa-Verlag, 2nd edition 1985, p.60.
The majority of solutions proposed hitherto have been directed to the use of so called internal mould release agents, which are incorporated into the plastic (precursor) mixture before injecting this into the mould and which produce an improved mould separation effect. In fact, some improvements have been produced with selected polyurethane plastics. A number of applications for using internal mould release agents have been disclosed. Examples of these can be found e.g. in Oertel (Publisher), Kunststoffhandbuch Volume VII, Polyurethane, published by Hanser-Verlag. Here again, the internal release agent often has to be supported by use of a small amount of external mould release agent and thus post-treatment, e.g. if the item is to be lacquered, is required. Internal mould release agents may also exude and thus reduce the long term performance. On the other hand, there are cases where no internal mould release agents can be used, for example in the case of metallic inserts which have to exhibit firm adhesion, e.g. foam-padded steering wheels.
Suggestions for improving the mould material have also been disclosed. The work of Menges et al. (Kunststofftechnik 13 (1974), No. 9/10, p. 181 et seq.), has disclosed that aluminium has substantially better release behaviour, that is less adhesion, for polythurethane than steel moulds. However aluminium moulds cannot be used for mass production because they are not sufficiently wear resistant. Ceramic moulds have also been suggested for improved separation, and siliconised, galvanised, nickel-plated, chrome-plated moulds and moulds coated with PTFE etc. have also been proposed. Moulds which have been provided with surface coatings can only be repaired, if at all, at great expense when they become damaged. None of these modifications to steel surfaces has been incorporated into an industrial scale process.
All in all there is still a high demand for polyurethane moulded items which have been produced using no mould release agents or at least very little mould release agent. Economic, ecologic and occupational hygiene reasons have been put forward as reasons for this.